fix: correct TypeScript types in ActivityService

Fixed two type errors:
- Changed ActivityType assertions from 'as ActivityType' to 'as const'
  for proper literal type inference
- Changed achievement.name to achievement.title to match the actual
  AchievementDefinition structure

This ensures type safety and proper union type discrimination.

api/src/app/services/activity.service.ts

@@ -89,7 +89,7 @@ export class ActivityService {
 
     return suggestions.map((suggestion) => ({
       id: `suggestion-${suggestion.id}`,
-      type: "SUGGESTION" as ActivityType,
+      type: "SUGGESTION" as const,
       user: suggestion.user as ActivityUser,
       entityType: suggestion.entityType,
       suggestionTitle: suggestion.title,
@@ -139,7 +139,7 @@ export class ActivityService {
         );
         return {
           id: `like-${like.id}`,
-          type: "LIKE" as ActivityType,
+          type: "LIKE" as const,
           user: like.user as ActivityUser,
           entityType: like.entityType,
           entityId: like.entityId,
@@ -229,7 +229,7 @@ export class ActivityService {
 
       return {
         id: `comment-${comment.id}`,
-        type: "COMMENT" as ActivityType,
+        type: "COMMENT" as const,
         user: comment.user as ActivityUser,
         entityType,
         entityId,
@@ -282,10 +282,10 @@ export class ActivityService {
         const achievement = ACHIEVEMENTS[ua.achievementKey];
         return {
           id: `achievement-${ua.id}`,
-          type: "ACHIEVEMENT" as ActivityType,
+          type: "ACHIEVEMENT" as const,
           user: ua.user as ActivityUser,
           achievementKey: ua.achievementKey,
-          achievementName: achievement.name,
+          achievementName: achievement.title,
           achievementIcon: achievement.icon,
           achievementPoints: achievement.points,
           createdAt: ua.earnedAt!,